Well I have a weird situation in here, and I personally feel that this may be connected to temp sensor on the CPU.

I have lately installed X9100 in a intel-based R500 motherboard. Everything works fine on stock voltage, the system runs cool under load (~65C with Orthos test) - however, once every couple of minutes I've got the readings of likes ~90C. Tried to measure with both HWinfo, tpfancontrol and Throttlestop, but all are pretty much the same. Fresh thermals applied, fan and heatpipe work perfectly fine as well. Any ideas?

I will check the CPU on a non-thinkpad unit. Actually this may be true since the power consumption on this particular one is huge as well. Anyway will try to unlock / overclock with extra voltage to seek for the temp jumps.

T9900 is signifficantly more expensive, not to mention multiplier lock. Or I will drop it back to P9600/P9700.

It's possible your CPU was damaged from excessive overclocking, or it could be a knock off. I guess the only way to know for sure is to compare it with a known good CPU. I can tell you that several years ago I built some R500 with an iMac CPU that was essentially similar to a T9900. Although they definitely ran warmer than the other CPU options, there was nothing worrisome and certainly never saw 90 degrees Centigrade.

It's possible your CPU was damaged from excessive overclocking, or it could be a knock off.

Bingo. Must have been either damaged before or counterfeit. Tested on HP 8730w, and it kills the system on stock clock and voltage while with far superior cooling system. I did not overclock it at any point before the test.

Yup, but these are no longer 16:10.
Not to mention you'd put the same RAM / SSD to the W510/520 - so the difference is actually CPU cost only
As for graphics, this particular one will most likely run on 2503 dock, so no big deal in there.

Anyway, let's just not talk about personal preferences. If this is the price thing only, I'm getting elitebook 8740w/8760w/8770w.